To create a new UIWindow over the main window

UIWindow *window1 = [[UIWindow alloc] initWithFrame:CGRectMake(0, 0, 320, 320)];window1.backgroundColor = [UIColor redColor];window1.windowLevel = UIWindowLevelAlert;[window1 makeKeyAndVisible];

Finally I know why it doesn't work, because window1 is a method var, and it will lost after the method executed. So I declare a new @property for it, as

@property (strong, nonatomic) UIWindow *window2;

and change the code like

UIWindow *window2 = [[UIWindow alloc] initWithFrame:CGRectMake(0, 80, 320, 320)];window2.backgroundColor = [UIColor redColor];window2.windowLevel = UIWindowLevelAlert;self.window2 = window2;[window2 makeKeyAndVisible];

it works!

Xcode 8 + Swift

class ViewController: UIViewController {    var coveringWindow: UIWindow?        func coverEverything() {        coveringWindow = UIWindow(frame: (view.window?.frame)!)                if let coveringWindow = coveringWindow {            coveringWindow.windowLevel = UIWindowLevelAlert + 1            coveringWindow.isHidden = false        }    }}

According to the documentation, to receive events that do not have a relevant coordinate value, such as keyboard entry, make it key instead of merely ! isHidden:

coveringWindow.makeKeyAndVisible()

You can even control the transparency of its background, for a smoke effect:

coveringWindow.backgroundColor = UIColor(white: 0, alpha: 0.5)

Note that such window needs to handle orientation changes.

Your window1 object is a local variable, when the code runs out of this method, this object does not exist any more. Any UIWindow object we created will be add to the [[UIApplication sharedApplication] windows], but this array only keeps a weak reference to any UIWindow object, so it's up to your own code to keep the window object existing. Why apple implemented it like this, I guess, is [UIApplication sharedApplication] object exists as long as the app runs, doing so to avoid keeping the UIWindow objects which only needs to exist for a while living in the memory "forever".

What's more, your code could run with MRC.